Here are two images of the textiles displayed as isosurface (top / face view)
:

Syncoglass 420R Vf 42 step 0-001

Syncoglass 420R Vf 42 step 0-1 ell

a

b

The problem probably come from the fact that the image size in Z is too small
compare to the aperture between the yarns. On the following image, the desired
equivalent diameter is the one described by the red circle, but because of the
small size in Z, all the space between the yarns is considered as a pore of
aperture equal to the diameter of the green circle.

Because of this problem, the computed permeability should be lower than the
real one (especialy in the case of image b). A solution would be to add an empty
space on both side of the image, or to add layers.